2.1 Outlines

Org is implemented on top of Outline mode. Outlines allow a
document to be organized in a hierarchical structure, which (at least
for me) is the best representation of notes and thoughts. An overview
of this structure is achieved by folding (hiding) large parts of the
document to show only the general document structure and the parts
currently being worked on. Org greatly simplifies the use of
outlines by compressing the entire show/hide functionality into a single
command, org-cycle, which is bound to the TAB key.